This invention relates in general to sensing and controlling cardiac activities in implantable devices including, but not limited to, bradycardia pacemakers and cardioverter defibrillators. More particularly, the invention concerns such a device in which an intracardiac impedance-derived parameter, Half Cycle Activity (HCA) is used as a key variable.
It is a well-recognized challenge to monitor accurately physical demands on cardiac support imposed by different levels of work or exercise a patient engages in (Barold et al., Clin Cardiol (1997), Vol 20 (8): 726-9; Alt, Am J Cardiol (1999), Vol 83(5B): 17D-23D). And yet, it is essential for a cardiac pacemaker to operate at a pacing rate that correlates with the workload presented. It is equally essential for a cardioverter defibrillator to determine hemodynamic status of a patient before diagnosing a life-threatening arrhythmia and exerting ventricular anti-tachy therapy (Johnston et al., Eur Heart J (1998), Vol. 19 (12):1879-88). A variety of physiologic or non-physiologic sensors have been designed during the last two decades to produce a signal directly related to the metabolic demand, including blood pH (U.S. Pat. No. 4,009,721), QT interval (U.S. Pat. No. 4,228,803), blood O2 saturation (U.S. Pat. No. 4,399,820), blood temperature (U.S. Pat. No. 4,543,954), pressure sensor (U.S. Pat. Nos. 4,899,752 and 5,105,819), piezoelectric crystal (U.S. Pat. Nos. 4,140,132 and 4,428,378), accelerometer (U.S. Pat. Nos. 5,235,237 and 5,383,473), micro-accelerometer (U.S. Pat. Nos. 5,423,883, 5,480,412, Rickards et al., The Multicenter PEA Study Groupxe2x80x94PACE (1996) 19:12 Pt1, 2066-2071), thoracic and intracardiac impedance (U.S. Pat. Nos. 3,593,718, 4,291,699, 4,773,401, 5,085,583, 5,235,976, 5,562,711 and 5,782,884) etc. These sensors are less than optimal and have led to little commercial success for various reasons, including difficulty to implement due to the need for specific hardware, lack of correlation between the signal and actual workload, lack of hemodynamic feedback information, slow response, and lack of robustness, i.e., high sensitivity to noise or artifacts inherent to the signal.
To resolve the above problems, the present invention is directed to an improved method and apparatus for monitoring hemodynamic activities in implantable cardiac devices. In particular, the present invention relates to cardiac pacemakers, cardioverter defibrillators and like devices that use a physiologic sensing parameter, Half Cycle Activity (HCA) to monitor and adjust cardiac activities.
In accordance with one aspect of the invention, the impedance-derived parameter HCA correlates closely with the workload and at the same time provides hemodynamic feedback information. Thus, it allows a pacemaker system to implement accurately an increase in hemodynamically driven pacing rate, as well as to limit an inappropriate decrease of driven pacing rate advised by another sensor such as an accelerometer. On the other hand, it determines the maximum pacing rate for the pacemaker, thus preventing hemodynamic compromise from occurring.
In accordance with another aspect of the invention, this intracardiac impedance-derived parameter HCA is resistant to noise or artifacts. Unlike other sensing parameters, HCA measurement is based on the whole impedance waveform; therefore, it is robust to local artifacts. Additionally, HCA is based on a baseline-corrected impedance signal; thus, it is much more resistant to baseline shifts than other impedance-derived parameters.
In accordance with yet another aspect of the present invention, the implementation of such parameter, in particular HCA, uses a single bipolar ventricular pacing and sensing lead in a pacemaker or defibrillator. Such a ventricular lead is connected to a sensing module to gather sensed physiological events. Specifically, an impedance reading module connected to the ventricular lead reads impedance signals, which is directed to the HCA microprocessor, where HCA computation takes place in real time. The result of HCA values is then fed back into a pacing controller module, which through a pacing module completes the adjustment to the cardiac stimulating events such as the pacing rates.
The present invention with all aspects of an impedance-derived parameter such as HCA, therefore, brings significant improvements in cardiac activity management devices.